低下

1. Origins, structure, and basic meaning

• Structure: 低下 is a noun read ていか, meaning a decline in a level, rate, efficiency, or other measure. It forms the verb 低下する.
• Basic usage: It can describe work efficiency declining because of drowsiness or fish slowing as water temperature falls.
  • 眠気で作業の能率が低下した。 says that work efficiency declined because of drowsiness; 水温の低下に合わせて魚の動きが鈍くなった。 says that the fish moved more slowly in step with the drop in water temperature.

2. Common pitfalls and similar expressions

• 能率が低下する
  • が marks work efficiency as what declined, and で marks drowsiness as the cause.
• 低下に合わせて
  • に合わせて links the change in the fish's movement to the falling water temperature.
• 低下 and 減少
  • 低下 lowers a level, performance, or measured value, while 減少 primarily reduces an amount or scale.